I'm sure it seems like a lot to know, I'd be in the same position going to English.
1. Are Full Quarter Horse bars the norm in Western saddles? My mare is very wide (XW in English AP saddle), but then again, I feel like many Western horses are...Would a not-Full QH bar saddle not fit? Aren't most Western saddles made to fit stocky QH types? I've watched videos on Western saddle fit but will look up some local saddle fitters because I am confused.
Full-QH and Semi-QH are the most common bars on western saddles. A Full-QH will probably fit fine so I would start there and only worry about the bars further if it clearly won't work. Quarter Horses come in a pretty large variety of shapes these days from big and tanky to not much bigger than an Arabian in the cutting blood lines.
Full-QH bars are popular with people who ride lots of different horses with one saddle.
2. What type of Western saddle should I look for if we are just occasionally trail riding and hacking around a few times a week? From what I have seen, "trail" saddles are more expensive? I have been looking at buying used (I've found a few threads on here with brand recommendations) OR getting a Corriente, as the pricing is reasonable and I've also read good recommendations for those on here. It seems like barrel saddles are cheaper, but I am not sure what type of Western saddle will be most comfortable for my horse and myself. There are so many types! Also with the Corrientes - I love the look of the lighter oil saddles, but do these lighter saddles darken quickly? I've only ever had dark brown or black tack. I'm not sure what's "in," but looking on Craigslist/eBay I mostly see dark oil tack.
I have never owned a Corriente or ridden one, but they have good reviews on horseforum. I typically recommend a Roohide cutting saddle to folks looking for a good used saddle at a reasonable price. They tend to have a good ground seat and are comfortable to ride for long hours especially once broke in.
The main differences in western saddles are the swells, horn, cantle and rigging and skirts. A trail saddle and barrel racing saddle tend to be pretty close in design with higher swells and a higher more upright cantle. You could say that some are basically the same design by a different name.
3. Which type of saddle pad would you all recommend for what I want to do? There are so many materials and types - again, very confused here. I assume it depends on the above questions. Wool? Foam? Those blanket-y ones? Idk.
The Diamond Wool Contoured Comfort Cutter pad is all I use. They are cheap enough to have several of them around and all you have to do when done riding is rinse them with a hose and leave them in the sun to dry. They are merino wool on the pad side, and sweat a horse very nice. https://nrsworld.com/products/contou...utter-pad-1-in
4. Breastplates - I have heard these are used to prevent the saddle from shifting or something like that. Is this true? Will a saddle with a good fit slip? Do I need one of these?
Breast collars are good for roping and riding up and down hills, and yes they are there to prevent the saddle slipping backwards. Barrel racers and trail riders use these sometimes for the same reason.
My personal favorite are "pulling collars". The main advantage of these is they sit above the horse's shoulder and don't interfere with shoulder movement. https://nrsworld.com/products/rougho...pulling-collar
5. What's the purpose of the back cinch thing? The cinch that straps behind the actual cinch? Do I need this?
It stops the back of the saddle from tipping up. For trail riding I would recommend it if you think you will be traveling up and down hills.
There is a hobble or keeper that links the front and rear cinch together. This is important and should be adjusted to hold the rear cinch in place. Without that keeper the rear cinch can slide back into the flank area, and that might turn your horse into a rodeo bronc :)
Just a little detail that isn't necessarily something you would know from looking at pictures.
6. Do most Western saddles have the set-up where you have to make that tie when connecting the cinch? I have no idea how to describe this, but I've seen ones where you have to tie the latigo and ones where they buckle in. I think I'd prefer the buckle one, but it seems like the tie is more common?
Buckles are more common these days on both sides. They aren't difficult to change and most western tack stores carry off side billets (right side) and latigos (left side) you can replace them with.